1. Field of the Invention
The present invention relates to a laser scanning system consisting of two plastic lenses to correct field curvature at a scanned surface, minimize focus shift caused by a temperature variation and degradation of optical performance due to a tilt error, and have relatively simplified surface shape facilitating mold processing and injection molding.
2. Description of the Prior Art
A conventional laser scanning system is constructed such that, as shown in FIG. 1, a light-modulated beam from a laser diode 1 is changed into a parallel collimating beam when going through a collimator lens 2 and then passes through a slit 3 elongated in the scan direction. The parallel beam having passed through slit 3 is incident on a cylinder lens 4 which has a refracting power in the sub-scan direction. After going through cylinder lens 4, the beam in the scan direction is incident on a reflecting surface of a polygonal rotating mirror 5 as the parallel beam to be deflected to a scanning lens system 6. Meantime, the beam in the sub-scan direction focuses on the reflecting surface of polygonal rotating mirror 5 to be deflected to scanning lens system 6.
The cylinder lens functions for minimizing inducing positional variation of a spot in the sub-scan direction when the beam forms an image on the scanned surface due to the fact that respective reflecting surfaces of polygonal rotating mirror 5 involve wobble slanted with respect to a rotating axis. Thus, Japanese Patent Publication No. Hei 4-110817 suggested a technique to minimize the influence created by wobble, by providing a toric surface having radii of curvature different from each other in the scan direction and sub scan direction in the scanning lens system to allow the reflecting surfaces of the polygonal rotating mirror and scanned surface 7 to accomplish an optical conjugate point in the sub-scan direction.
The above Patent Publication No. Hei 4-110817 attains favorable performance in spot size, linearity, etc. However, the overall lens surface consists of the toric surface apt to degrade results due to decenter and tilt error of the scanning lens surfaces.
In U.S. Pat. No. 4,639,072, a cylinder lens is disposed in the vicinity of a scanned surface 7 to inhibit the adverse effect due to the wobble. The scanning lens system satisfies f.multidot..theta. feature, i.e., ##EQU1## (refer to FIG. 2) while allowing the beam deflected by the polygonal rotating mirror to form an elliptical spot having its long axis in the sub scan direction at the scanned surface.
However, the above-stated optical conjugate point is not maintained due to the change of the deflecting surface when the polygonal rotating mirror is rotated to cause a phenomenon that an image forming spot in the sub scan direction appears to be asymmetrical right and left across the scanned surface. For this reason, U.S. Pat. No. 5,488,502 suggested a method for minimizing the positional variation of the image forming point of the scanned surface in the sub scan direction by forming a scanning lens adjacent to the scanned surface formed with a deformed cylinder lens whose radius of curvature is asymmetrical right and left in the sub scan direction. Also, in U.S. Pat. No. 5,488,502, a distance from the deflecting surface to scanned surface is shorter than that of the typical optical system, but the distance between lenses becomes greater for the purpose of making the characteristics and field curvature satisfactory to be troublesome in miniaturizing the unit.
Generally, the unit can be miniaturized when a ratio between a focal distance f and length L from the deflection point to the upper plane has a relation that L/f&lt;1.33 or so while maintaining the high optical performance, wide angle of view and excellent f.multidot..theta. characteristics. Whereas, the aberration is difficult to be corrected as further reaching to both ends rather than the center portion of the scanned surface to induce a problem of forming the spot size to be larger, thereby degrading the performance at both ends of the scanned surface owing to environment change, fabricating the lens and unit arrangement error, etc.